The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with single-photon emission computed tomography (SPECT) systems which include one or more transmission radiation line sources and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other types of nuclear medicine and transmission radiation diagnostic imagers.
Heretofore, single photon emission computed tomography has been used to study the radionuclei distribution in subjects. Typically, one or more radiopharmaceuticals were injected into a patient's blood stream for imaging the circulatory system or specific organs which absorb the injected radiopharmaceuticals. One or more gamma or scintillation camera heads were placed closely adjacent to a surface of the patient to monitor and record radiation emitted by the radiopharmaceuticals. In single photon emission computed tomography, the heads were rotated or indexed around the subject to monitor the emitted radiation from a multiplicity of directions. The data monitored from the multiplicity of directions was reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the patient.
One of the problems with the SPECT imaging technique is that the patient is not completely homogeneous in terms of radiation attenuation or scatter. Rather, the human patient includes many different tissue and bone types which absorb or scatter radiation from the radiopharmaceuticals to different degrees. The SPECT images can be made more accurate if they are corrected for the radiation lost to scattering or attenuation along each path through the human torso.
As described in our parent U.S. Pat. No. 5,210,421, a radiation line source can be positioned opposite one or more of the gamma or scintillation camera heads. Transmission radiation from the line source received by the opposite detector head can be reconstructed into a three-dimensional image representation of the radiation absorptive properties of each incremental volume element of the patient, analogous to a CT scan. This radiation attenuation information is utilized to correct the SPECT data. When the radiation line source and the radiopharmaceuticals have distinctly different energy peaks, the transmission radiation and photon emission radiation image data can be collected concurrently and separated based on energy.
One concern of the prior art line sources was in operator safety. The line sources typically include a tube filled with a radionucleide material which is emitting radiation continuously. There is no radiation generating "chemical reaction" that can be started and stopped. The prior art line sources lacked a fail-safe system for assuring that the operator would not be subject to unnecessary radiation, particularly while the line source was being mounted to or removed from the scanner.
There has also been a disposal problem with spent radiation line sources. The strength of the radiation source decreases exponentially with time. At the half-life of the radioisotope, the radiation source is about 1/2 strength. Typically, after a 50% reduction in line source strength, about one half-life, the radioisotope tube was replaced.
In the prior art, collimators were typically constructed of lead. When lead is struck with incident radiation, such as the radiation from the line source, the lead emits an x-ray with a characteristic energy of about 88 keV. The 88 keV x-ray has an energy which is sufficiently close to the emission energy of some common radiopharmaceuticals that it is difficult to distinguish the two. This inability to distinguish reliably radiation from the radiopharmaceuticals and radiation emitted from the lead caused errors in the resultant radiopharmaceutical image.
One solution for separating the lead x-rays coming from the line source assembly from the radiation emitted from the radiopharmaceuticals was to conduct the line source transmission radiation examination first. The transmission line source was removed or closed before the radiopharmaceuticals were injected and imaged. However, performing the transmission examination and the radiopharmaceutical diagnostic examination sequentially lead to registration problems. The transmission radiation data and radiopharmaceutical images data frequently became at least partially misaligned. This misalignment caused incorrect transmission radiation based corrections on the radiopharmaceutical data causing further image degradation.
The present application provides a new and improved transmission line source assembly which overcomes the above-referenced problems and others.